Writing multiple levels in a phase change memory

ABSTRACT

Structures and methods for a multi-bit phase change memory are disclosed herein. A method includes establishing a write-reference voltage that incrementally ramps over a write period. The increments of the write-reference voltage correspond to discrete resistance states of a storage cell of the multi-bit phase change memory.

FIELD OF INVENTION

The present disclosure relates to phase change memory devices. Morespecifically, the present disclosure relates to a PCM architecture andprocesses for storing information within that architecture.

BACKGROUND

Phase-change memory (a.k.a., “PCM” or “PRAM”) is a type of nonvolatilememory that stores information using phase change materials. Phasechange materials can change between a crystalline state and amorphousstate. The crystalline state has a low electrical resistance incomparison to the amorphous state. To change from the crystalline stateto the amorphous state, a current is passed through a phase changematerial to melt it via internal joule heating, and then a quench isperformed. A change from the amorphous state to the crystalline stateinvolves driving a phase change material to electrical breakdown, andannealing it using electrical current. Data can be stored and read fromthe phase change material based on its programmed electrical resistance.

By the above process, multiple bits of information may be stored in asingle storage cell of a phase change material to provide a “multi-bitphase change memory” (i.e., “multi-bit PCM”). In a multi-bit PCM, eachstorage cell can be set with a resistance state selected from a rangecorresponding to various intermediate states between the crystallinestate and the amorphous state. For example, different resistance states(e.g., R1, R2, R3, and R4) can represent to different binary values(e.g., 00, 01, 10, and 11).

Current implementations of multi-bit PCMs use a combination ofanalog-to-digital converters (ADCs) and digital to analog converters(DACs). However, these implementations are very slow for reading andeven slower for writing. In addition, the ADCs and DACs require a largeamount of chip area.

SUMMARY

In an aspect of the invention, there is a method for a multi-bit phasechange memory. The method includes establishing a write-referencevoltage that incrementally ramps over a write period. Increments of thewrite-reference voltage correspond to discrete resistance states of astorage cell of the multi-bit phase change memory.

In another aspect of the invention there is a method for storinginformation in a multi-bit phase change memory. The method includeswriting an initial write value in a storage cell of the multi-bit phasechange memory. The storage cell has a predetermined range of discreteresistance states. The initial write value is a predetermined value atthe midpoint of the range. The method also includes reading a valuestored in the storage cell by the writing. The method further includescomparing the value stored in the storage cell to a target value.Additionally, the method includes adjusting the value stored in thestorage cell based on the comparing.

In another aspect of the invention there is a phase change memory systemcomprising a storage cell, a read head, and a write head. The read headretrieves information stored in the storage cell using a read-referencevoltage that incrementally ramps over a read period. The write headstores information in the storage cell using a write-reference voltagethat incrementally ramps over a write period.

In another aspect of the invention, a design structure tangibly embodiedin a machine readable storage medium for designing, manufacturing, ortesting an integrated circuit is provided. The design structurecomprises structures associated with a predefined minimum feature sizeand a predefined minimum spacing size. In further embodiments, ahardware description language (HDL) design structure encoded on amachine-readable data storage medium comprises elements that whenprocessed in a computer-aided design system generates amachine-executable representation of the multi-bit PCM of the presentinvention. In still further embodiments, a method in a computer-aideddesign system is provided for generating a functional design model ofthe multi-bit PCM of the present invention. The method comprisesgenerating a functional representation of the multi-bit PCM of thepresent invention.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The present invention is described in the detailed description thatfollows, in reference to the noted plurality of drawings by way ofnon-limiting examples of exemplary embodiments of the present invention.

FIG. 1 shows an exemplary read head of a multi-bit PCM in accordancewith aspects of the invention;

FIG. 2 illustrates a read operation by an exemplary read head of amulti-bit PCM in accordance with aspects of the invention;

FIG. 3 shows an exemplary write head of a multi-bit PCM in accordancewith aspects of the invention;

FIG. 4 illustrates a write operation by an exemplary write head of amulti-bit PCM in accordance with aspects of the invention;

FIG. 5 shows an exemplary system for a multi-bit PCM in accordance withaspects of the invention;

FIG. 6 illustrates a process for translating a value in a multi-bit PCMin accordance with aspects of the invention;

FIGS. 7 and 8 show an exemplary process flow for storing an input valuein a multi-bit PCM in accordance with aspects of the invention;

FIG. 9 illustrates a write process by an exemplary system in accordancewith aspects of the invention;

FIGS. 10-22 illustrate steps of an exemplary write process in accordancewith aspects of the invention; and

FIG. 23 shows a flow diagram of a design process used in semiconductordesign, manufacture, and/or test.

DETAILED DESCRIPTION

The present disclosure relates to PCMs. More specifically, the presentdisclosure relates to a PCM architecture and processes for storinginformation within that architecture. A PCM architecture in accordancewith aspects of the present invention writes information in PCM storagecells by establishing a current ramp which changes its value over apredetermined write time period. The write time period is broken intoequal time increments (e.g., 64 increments of 1.5 ns), wherein eachincrement corresponds to a digital write current value. A target valueto be stored in the storage cell is compared to the digital writecurrent value. At an increment when the values are equal, the rampedcurrent is directed through the storage cell, which stores the targetvalue in the storage cell by changing its physical state and, thereby,its electrical resistance.

In a read operation, a current ramp generates a read-reference voltagefor a sense comparator over a predetermined read time period. The readtime period is broken into the same number of time increments as thewrite time period. When the voltage across the storage cell equals thereference voltage, the sense comparator triggers. The time increment atwhich the sense comparator triggers corresponds to the value in thestorage cell.

Due to physical and operational variations among storage cells of PCMs,storing information in a particular storage cell can involve multipleread and write iterations to adjust the stored information such that theactual value stored by the storage cell (i.e., a read value) reflectsthe intended value (i.e., a target value). In other words, multiplewrite iterations may be performed to provide sufficient certainty that avalue written to a particular storage cell matches the value that willbe read from that cell. In accordance with aspects of the invention, thewrite operation is self-calibrating. That is, the write operationperforms an iterative process wherein it writes a first, nominal valuein the center of the expected write range of the storage cell (e.g., 32in a range 0-63). A read operation follows the write operation todetermine whether the value read in the storage cell is lower than thetarget value (e.g., lower than 32). A number of subsequent write/readoperations are performed that compare the intermediate values stored bypreceding write iterations with the target value. By doing so, the writeoperation rewrites the cell over a number of iterations such that thefinal value stored is reliably an input value.

Additionally, in accordance with aspects of the invention, the multi-bitPCM architecture uses only DACs for performing read and writeoperations, without using ADCs for reading and writing. Based on digitalinputs, a DAC generates a reference current for the read operation and aDAC (which may be the same or different than the first) generates areference current for the write operation. In embodiments, the DACsoutput current ramps that generate reference voltages for the read andwrite operations. The lack of ADC's in the architecture reduces the areaof PCMs in semiconductor chips. For example, a multi-bit PCM inaccordance with the instant invention can be implemented a chip areathat is 10% smaller or less than a conventional PCM that employs ADCs.

FIG. 1 shows an exemplary read head 100 for reading information from astorage cell 105 of a multi-bit PCM in accordance with aspects of theinvention. The read head 100 includes a sense comparator 110 having afirst input 112, a second input 114, and an output 116. The sensecomparator 110 can be, in embodiments, a device adapted and/orconfigured to change logic states (e.g., from a high logic state to alow logic state) when the first input 112 and the second input 114 arethe same or substantially the same (e.g., within about 10 mV of oneanother).

The first input 112 of the sense comparator 110 receives aread-reference voltage 118 that is ramped (e.g., increases) over apredetermined read period. The read-reference voltage 118 can begenerated by a controllable current source 120. In accordance withaspects of the invention, the controllable current source 120 is a DACand the read head 100 does not include any ADCs. The DAC is configuredand/or adapted to generate a current that is incrementally ramped over apredetermined range (e.g., 0.5 V to 1.5 V), wherein the increments areequal divisions (i.e., time intervals) of the read period (e.g., 100ns). Each of the increments corresponds to a discrete resistance stateof the storage cell 105. In embodiments, the read-reference voltage 118is ramped in 64 increments having a respective voltage level (e.g., a 15mV step) that corresponds to 64 different resistance states of thestorage cell 105. For example, at the 40th time increment, the 40thvoltage step of the read-reference voltage 118 can correspond to a 40thresistance state of the storage cell 105. Based on this correspondence,a value stored in the storage cell 105 is retrieved.

The second input 114 of the sense amplifier 110 is electricallyconnected to the storage cell 105. The storage cell 105 maybe associatedwith conventional memory lines so that it can be selected within anarray of storage cells for reading and writing. More specifically, thestorage cell 105 may be associated with a bit line (“BL”) 135, a wordline (“WL”) 137, a master bit line (“MBL”) 140, and a column decoderline (“CD”) 145. The word line 137 selectively connects the storage cell105 to ground voltage via transistor 155. The column decoder 145selectively connects the storage cell 105 and the bit line 135 to themaster bit line 140 via transistor 165. The storage cell 105 is selectedfor reading by the bit line 135, the word line 137, and the columndecoder 145 such that a voltage corresponding to the value of thestorage cell 105 is produced on master bit line 140 due to the voltageacross the storage cell 105 based on its resistance state. Because theresistance state of the storage cell 105 is selectively set to one of anumber of predefined values, its resistance can be interpreted asinformation (e.g., a combination of data bits).

In accordance with aspects of the invention, the read head 100 performsa read operation as follows. Upon initiation of the read operation, thecurrent source 120 incrementally ramps the read-reference voltage 118from a first predetermined voltage to a second predetermined voltageover a predetermined read period (i.e., time). At one of the timeincrements of the read-reference voltage 118, a voltage on the masterbit line 140 at the input of the sense comparator 114 (due to the bitline 135 and the resistance of the storage cell 105) is the same orsubstantially the same (e.g., ±10 mV) as the voltage at the input 112(due to the current source 120 and the resistance across resistor 122).At that time increment, a output signal 150 at the output 116 sensecomparator 110 changes logic state (e.g., from high to low). The valueof the time increment indicates the resistance state of the storage cell105. That is, the increment of the read-reference voltage 118 at whichthe sense comparator 110 changes logic state (i.e., triggers)corresponds to the value stored in the storage cell 105. For example,the current source 120 can ramp-up in 64 time increments (i.e., 0-63).If the read output value 150 at output 116 changes logic states at timeincrement 40, the value stored in the storage cell 105 would beinterpreted to be 40.

FIG. 2 illustrates a read operation 200 for a storage cell (e.g.,storage cell 105) performed by a read head (e.g., read head 100) inaccordance with aspects of the invention. Line 202 represents aread-reference voltage (e.g., read-reference voltage 118) during anexemplary read process. Line 203 represents a predetermined read periodover which the read-reference voltage is incrementally ramped. Line 204represents an output signal (e.g., output signal 150) of a sensecomparator (e.g., sense comparator 110) that compares the read-referencevoltage and a voltage across the storage cell. Line 208 represents therange of resistance states to which the storage cell can be programmed(i.e., recorded or written).

In accordance with the exemplary embodiment shown in FIG. 2, theread-reference voltage (i.e., line 202) is ramped-up in 64 steps (i.e.step 0 to step 63) from a first voltage V1 (e.g., 0.5 V) to a secondvoltage V2 (e.g., 1.5 V). The increments 0-63 of the read-referencevoltage correspond to the 64 time increments (i.e., time increments0-63) of the time range (i.e., line 203) in the 64 resistance states ofthe storage cell (i.e., line 208). In embodiments, each time incrementcorresponds to a step-wise increase (e.g., by current source 120) in theread-reference voltage. When the read-reference voltage and the voltageacross the storage cell are the same, or substantially the same (e.g.,±10 mV) at time increment 37, the sense comparator triggers. As such,its output (i.e., line 204) switches from a first logic level V3 (e.g.,1.0 V) to a second logic level V4 (e.g., 0.0V), which corresponds toprogrammed resistance state 225 of the storage cell.

In accordance with aspects of the invention, the storage cell can beprogrammed to have one of a number of discrete resistance states. Asindicated by divisions 0-63 of line 208, embodiments of the storage cellhas 64 (i.e., 0-63) resistance states. The divisions 208 of the 64discrete resistance states can be grouped into ranges (i.e., buckets) ofvalues 210, 212, 214, and 216. Each of the ranges of values 210, 212,214, and 216 can correspond to a value stored in the storage cell 105.For example, in the example shown in FIG. 2, resistance states 0 . . .15 correspond to binary value 00, resistance states 16 . . . 31correspond to binary value 01, resistance states 32 . . . 47 correspondto binary value 10, and resistance states 48 . . . 63 correspond tobinary value 11. It is understood that the number of steps and thenumber of values, as well as their respective ranges and divisions shownin FIG. 2 are provided for example, and that other steps, values, anddivisions can be used in embodiments of the present invention.

Still referring to FIG. 2, the 64 steps of the read-reference voltage(i.e., line 202) respectively correspond to the 64 time increments ofline 208 and the 64 resistance states 208 of the storage cell. Thus, bydetermining the time increment at which the output of the sensecomparator switches from voltage level V3 to voltage level V4, theresistance state 225 of the storage cell can be determined and thecorresponding one of the values 210, 212, 214, and 216 can beinterpreted. In the example shown in FIG. 2, the resistance state 225 ofthe storage cell has a value of 37. Prior to time increment 0, theoutput of a sense comparator is initialized to the voltage level V3. Attime increment 0, a read operation begins and the read-reference voltageis incrementally ramped from voltage V1 to voltage V2 in steps 0 to 63.During the voltage increments 0-36 (and correspondingly time increments0-36), there is no match between the read-reference voltage and voltageacross the storage cell. As such, the output of the sense comparatorremains at the first voltage level V3 during time increments 0-36.However, at voltage increment 37 (and, correspondingly, time increment37), the sense comparator determines that the read-reference voltage isthe same or substantially the same (e.g., ±10 mV) as the voltage due tothe resistance state 225 of the storage cell. As a result, the output ofthe sense comparator changes from voltage level V3 to voltage level V4.Since this event occurs at time increment 37, the resistance state 225of the storage cell can be interpreted to be 37, which is in the range214 that corresponds to a stored value of 10.

FIG. 3 shows an exemplary write head 300 for storing information in amulti-bit PCM in accordance with aspects of the invention. The writehead 300 includes a buffer 310 having a first input 312, a second input314, and an output 316. The buffer 310 can be, in embodiments,configured and/or adapted as a unity gain amplifier by tying the output316 to the second input 314. The first input 312 of the buffer 310receives a write-reference voltage 318 that incrementally ramps (e.g.,increases) from a first voltage to a second voltage over a predeterminedwrite period. The write-reference voltage 118 can be generated by acontrollable current source 320. In accordance with aspects of theinvention, the controllable current source 320 is a DAC and the writehead 300 does not include any ADCs. The DAC is configured and adapted togenerate a current that incrementally ramps-up over a predeterminedrange (e.g. 0.0 V to 1.5 V), wherein the increments are equal timedivisions (e.g., 1.5 ns) of the write period (e.g., 100 ns). Each of theincrements corresponds to one of the discrete resistance states of thestorage cell 105. Additionally, at the output 316 of the buffer 310,there are transistors 165, 324, 330, 332, 336, and 338 that togetherselectively provide a write current 340, the store a value in thestorage cell 105.

In accordance with aspects of the invention, writing with differentcurrent levels over a wide range using identical pulse widths will, overa desired range, yield monotonic results with larger currents causinglarger values of resistance to be given to the storage cell 105. Duringa write operation, writing is initiated by a write enable signal 339,the bit line 135, the word line 137, and the column decoder 145 in aconventional manner. The write-reference voltage 318 is ramped over thepredetermined range by the current source 320. A write signal 345provided to transistor 324 pulses during a specific write current rampstep to write the storage cell 105 to a corresponding resistance state.The pulse of the write signal 345 can be triggered based on a clock or acounter at the target value corresponding to an increment of thewrite-reference voltage 318. For example, a clock or a counter may counttime increments from (e.g., 0-63) corresponding to the steps of thewrite-reference voltage (e.g., 0-63), and trigger the pulse at aparticular time increment (e.g., 32) corresponding to a target voltage(e.g., 0.75 V).

In accordance with aspects of the invention, the write head 300 isconfigured and/or adapted as a current mirror via transistors 332 and336, wherein the current driven through transistor 338 is reflected as awrite current 340 that is driven through the storage cell 105. The flowof write current 340 can be controlled by the word line 137. That is,the word line 137 is either on or off (1 V or 0 V) to control transistor155 as a switch.

FIG. 4 illustrates an exemplary write operation 400 for a storage cell(e.g., storage cell 105) performed by a write head (e.g., write head300) in accordance with aspects of the invention. Line 402 represents awrite-reference voltage (e.g., write-reference voltage 318) that isramped over a predetermined number steps (e.g., 64 steps) from a firstvoltage V5 (e.g., 0.0 V) to a second voltage V6 (e.g., 1.5 V). Line 403represents a predetermined write time period over which thewrite-reference voltage is ramped. For example, the write time may bekept by a clock or counter, wherein each increment 0-63 of the writeperiod corresponds to a step-wise increase (e.g., by current source 320)in the write-reference voltage. Line 404 represents a write signal(e.g., write signal 345), that pulses at a particular time increment tostore a value in the storage cell by setting a corresponding resistancestate of the storage cell. Line 208 represents the range of resistancestates to which the storage cell can be set (i.e., recorded or written)based on the write-reference voltage.

In the example shown in FIG. 4, a target resistance state 425 of thestorage cell has a value of 35. Initially, the write signal (i.e., line404) is in a low logic state (e.g., 0.0 V) as write-reference voltage318 (i.e., line 402) ramps from V5 to V6 over steps 0-63 correspondingto time increments 0-63. At time increment 35 and step 35 of thewrite-reference voltage, which correspond to the value to be recorded inthe storage element, the write signal pulses from a low logic state V7(e.g., 0.0 V) to a high logic state V8 (e.g., 1.0 V) to store the valuein the storage cell. As is evident in FIG. 4, the resistance state 425of step 35 is in range 214 of the 64 resistance states of the storagecell (i.e., line 208), which corresponds to the binary value 10 beingstored in the storage cell.

FIG. 5 shows an exemplary system 500 for writing information to andreading information from the storage cell 105 in accordance with aspectsof the present invention. The system 500 stores a value of an inputsignal 504 to the storage cell 105 in an iterative process of writingand reading. The system 500 includes the storage cell 105, the read head100, the write head 300, input/output pins 503, a write head controller505, a read head controller 510, and a column decoder (“CD”) multiplexer535.

While the example shown in FIG. 5 includes one storage cell 105,multi-bit PCMs in accordance with aspects of the invention can includelarge, addressable arrays of such storage cells that are written andread by write control heads and read control heads which are the same orsimilar to write head controller 505 and read head controller 510.

The input/output pins 503 correspond to the storage element 105. Inembodiments, there are two input/output pins 503 that that receive theinput value from, e.g., a memory controller, and provide it as the inputsignal 504 to the write head controller 505. Memory controllers thatsend and receive information from memory are conventional in computerprocessing systems and are understood by those having ordinary skill inthe relevant art such that further explanation is not required for suchartisans to make and use the invention. In embodiments, the input signal504 includes 2-bits of data corresponding to one of four possible inputvalues (e.g., 00, 01, 10, and 11 in binary). It is understood, however,that embodiments of the invention are not limited to two pins or 2-bitsof data, and that other embodiments may user more pins (e.g., 3 or morepins that carry 3 or more bits of data). Further, a number of storagecells can be used together to store words that are greater than 2-bits.For example, four storage cells may each store 2-bits of an 8-bit word.

The write head controller 505 is a device, software, or a combinationthereof that determines a target value for the storage cell 105 based onan input value provided by the input signal 504. The target value isused to control the writing of the storage cell 105 to one of the rangeof resistance states by the write head 300. In accordance with aspectsof the invention, each target value corresponds to at least one of theresistance states of the storage cell 105. In embodiments, the range ofresistance states of the storage cell 105 is greater than the range ofinput values. For example, the range of resistance states can include 64discrete resistances 000000 to 111111 in binary notation (i.e., 0-63 indecimal notation), and the range of input values can include values 00to 11 (i.e., 0-3 in decimal notation). Thus, each resistance state ofthe storage cell 105 can correspond to a 6-bit value, whereas each inputvalue can correspond to a 2-bit value. Accordingly, in embodiments, thewrite head controller 505 translates the 2-bit input value to a 6-bitvalue. For example, the write head controller 505 can convert the inputvalues 00, 01, 10, and 11 (i.e. 0, 1, 2, and 3 in decimal notation) totarget values of 001000, 011000, 101000, and 111000 (i.e., 8, 24, 40,and 56 in decimal notation), respectively. By providing a range ofresistance states for each input value, the system 500 provides a marginof error between the different input values that may be stored in thestorage cell 105. In embodiments, the write head controller 505translates the input value to a target value by adding the receivedinput value with the bits 1000. For example, an input value of 10 can beadded to provide a target value of 101000 (i.e., 40 in decimal).However, other methods of translation may be used in embodiments of theinvention. For example, the write head controller 505 may use a lookuptable that maps input values to target values.

Additionally, the write head controller 505 determines a write value ofthe write signal 345 for writing to the storage cell 105. In accordancewith aspects of the invention, the write head controller 505 triggers apulse of the write signal 345 based on the target value (e.g., in amanner similar to that described with respect to line 404 in FIG. 4).The write head controller 505 can determine the write value based on thetarget value based on a write value from a previous write iterationduring a write process. For example, the write control 505 headincreases or decreases the write value from an immediately precedingwrite iteration based on comparison of a value read from the storagecell 105 after the immediately preceding write iteration and the targetvalue. By doing so, the system 500 iteratively adjusts the target valuewritten to the storage cell 105 over several write iterations.

Still referring to FIG. 5, the read head controller 510 is a device,software, or a combination thereof that receives the read output signal150 from of the read head 100 and determines a value of the informationstored in the storage cell 105. In embodiments, the read output signal150 of the read head 100 changes states when the current step of theread-reference voltage 118 (e.g. 0-63) corresponds to the resistancestate (e.g., 0-63) of the storage cell 105. For example, a comparator(e.g., comparator 110) triggers when the ramped read-reference voltage118 is the same (or substantially the same) as the voltage across thestorage cell 105 due to its programmed resistance state. Thus, based onthe time at which the read output signal 150 changes states (e.g., 35),the read head controller 510 determines (i.e., retrieves) thecorresponding value stored by the storage cell 105 (e.g., 35).

Additionally, in accordance with aspects of the invention, the read headcontroller 510 provides a read signal 515 to the write head controller505 that feeds back the value of the storage cell 105 for use in thedetermining write values in iterations of a write process. Inembodiments, the read head controller 510 provides only the mostsignificant bits (e.g., the three most significant bits) of the value ofthe storage cell in the read signal 515. Additionally, the read headcontroller 510 provides the read signal 515 to the input/output pins 503as an output to, e.g., a memory controller. In embodiments, the readhead controller 510 provides only the most significant bits (e.g., thetwo most significant bits) in the read signal 515, which represent a3-bit value (e.g., 00, 01, 10, and 11) of the value stored in thestorage cell 105.

The write head 300 is a device, software, or a combination thereof thatwrites information to the storage cell 105 by programming it with aselected resistance state. The write head 300 may be the same or similarthe one shown in FIG. 3. During a write operation within a writeprocess, the write head 300 generates ramped write reference voltage 318that provides the write current 340 to the storage cell 105. The writesignal 345 triggers the write current 340, which stores the target valuein the storage cell 105 based on the value of the write referencevoltage 318 at the write value determined by the write head controller505. For example, when the write value is 40, the write head controller505 controls the write signal 345 to pulse at voltage step 40 of thewrite-reference voltage 318. The write head controller 505 may determinethe timing of the pulse for the write signal 345 based on a write clocksignal 520 that drives a counter in the write head controller, whereinthe counter is synchronized with the steps of the write-referencevoltage 318. In embodiments, both the write head controller 505 and thewrite head 300 receive the write clock signal 520 such that a countermaintained by the write head controller 505 is synchronized with thewrite-reference voltage 318 (i.e. time increments 0-63 correspond tovoltage steps 0-63).

The read head 100 is a device, software, or a combination thereof thatreads the storage cell 105 by determining its resistance state. The readhead 100 may be the same or similar to the read head 100 in FIG. 1.During a read operation within a write process, the read head 100applies a voltage to the master bit line 140, and compares voltageacross the storage element 105 due to its programmed resistance with theramped read-reference voltage 118. That is, the output signal 150 of theread head 100 changes logic states when these two voltages are the same.The read head controller 510 determines the value stored in the storagecell 105 based on a time increment (e.g. 35 in decimal) at which theread head output signal 150 changes states (i.e., triggers). Todetermine the time increment, the master read head 510 can receive aread clock signal 525 that drives a counter maintained by the read headcontroller 510, and that is synchronized with the time increments of theread-reference voltage 118 based on read clock signal 525. Inembodiments, both the read head controller 510 and the read head 100receive the read clock signal 525 such that a counter maintained by theread head controller 510 is synchronized with the read-reference voltage318 (i.e. time increments 0-63 correspond to voltage increments 0-63).

The column decoder line multiplexer (i.e., “CD Mux”) 535 includestransistor 165 that receives column decoder line 145. Additionally,column decoder line multiplexer 535 can include control logic thatselectively controls electrically connections of the read head 100 andthe write head 300 to the storage cell 105 such that they are notconnected at the same time. For example, the control logic may connectthe write head 300 or the read head 100 based on the state of a writeenable signal (e.g., write enable signal 339) and the read enable signal550.

FIG. 6 illustrates a process for translating an input value to a targetvalue in accordance with embodiments of the invention. In embodiments,the translation is performed by a device (e.g., write head controller505) using hardware, software, or a combination thereof. In embodiments,2-bit input value (e.g., from input signal 504) is stored in a storagecell (e.g., storage cell 105) that is configured and/or adapted to have64 discrete resistance states (e.g., 000000-111111 in binary notation),wherein each resistance state is associated with a target value (e.g.,000000-111111 in binary notation). As such, the 2-bit input value (i.e.,00, 01, 10, and 11 in binary notation) is translated to a 6-bit targetvalue, which corresponds to one of the 64 discrete resistance states.

In accordance with aspects of the invention, each of the four inputvalue 00, 01, 10, and 11 is mapped to a value at the midpoint of acorresponding range of the resistance states. In embodiments, these fourranges are 000000-001111 (i.e., 0-15 in decimal notation), 010000-011111(i.e., 16-31 in decimal notation), 100000-101111 (i.e., 32-47 in decimalnotation), and 110000-111111 (i.e., 48-63 in decimal notation). Themidpoint of each range is 001000 (i.e., 8 in decimal notation), 011000(i.e., 24 in decimal notation), 101000 (i.e., 40 in decimal notation),and 111000 (i.e., 56 in decimal notation), respectively. That is, 00 ismapped to 001000; 01 is mapped to 011000); 10 is mapped to 101000; and11 can be mapped to 111000. Doing so provides a 16-bit margin for errorsaround each of the four target values.

In accordance with aspects of the invention, the most significant bitsof each range are the same as the input value. For the input value of00, the two most significant bits are in the range 000000-001111 are 00.For the input value of 01, the two most significant bits in the range010000-011111 are 01. For the input value of 10, the two mostsignificant bits in the range 100000-101111 are 10. For the input valueof 11, the two most significant bits in the range 110000-111111 are 11.Thus, there is a direct correspondence between a 2-bit input value andthe two most significant bits of the ranges of resistance states. Assuch, translation from a resistance range to a 2-bit value can beperformed by determining the two most significant bits.

As will be appreciated by one skilled in the art, aspects of the presentinvention may be embodied as a system, method or computer programproduct. Accordingly, aspects of the present invention may take the formof an entirely hardware embodiment, an entirely software embodiment(including firmware, resident software, micro-code, etc.) or anembodiment combining software and hardware aspects that may allgenerally be referred to herein as a “circuit,” “module” or “system.”Furthermore, aspects of the present invention may take the form of acomputer program product embodied in one or more computer readablemediums having computer readable program code embodied thereon.

Any combination of one or more computer readable medium(s) may beutilized. The computer readable medium may be a computer readable signalmedium or a computer readable storage medium. A computer readablestorage medium may be, for example, but not limited to, an electronic,magnetic, optical, electromagnetic, infrared, or semiconductor system,apparatus, or device, or any suitable combination of the foregoing. Morespecific examples (a non-exhaustive list) of the computer readablestorage medium would include the following: an electrical connectionhaving one or more wires, a portable computer diskette, a hard disk, arandom access memory (RAM), a read-only memory (ROM), an erasableprogrammable read-only memory (EPROM or Flash memory), an optical fiber,a portable compact disc read-only memory (CD-ROM), an optical storagedevice, a magnetic storage device, or any suitable combination of theforegoing. In the context of this document, a computer readable storagemedium may be any tangible medium that can contain or store a programfor use by or in connection with an instruction execution system,apparatus, or device.

Computer program code for carrying out operations for aspects of thepresent invention may be written in any combination of one or moreprogramming languages, including an object oriented programming languagesuch as Java, Smalltalk, C++ or the like and conventional proceduralprogramming languages, such as the “C” programming language or similarprogramming languages. The program code may execute entirely on theuser's computer, partly on the user's computer, as a stand-alonesoftware package, partly on the user's computer and partly on a remotecomputer or entirely on the remote computer or server. In the latterscenario, the remote computer may be connected to the user's computerthrough any type of network, including a local area network (LAN) or awide area network (WAN), or the connection may be made to an externalcomputer (for example, through the Internet using an Internet ServiceProvider).

Aspects of the present invention are described below with reference toflowchart illustrations and/or block diagrams of methods, apparatus(systems) and computer program products according to embodiments of theinvention. It will be understood that each block of the flowchartillustrations and/or block diagrams, and combinations of blocks in theflowchart illustrations and/or block diagrams, can be implemented bycomputer program instructions. These computer program instructions maybe provided to a processor of a general purpose computer, specialpurpose computer, or other programmable data processing apparatus toproduce a machine, such that the instructions, which execute via theprocessor of the computer or other programmable data processingapparatus, create means for implementing the functions/acts specified inthe flowchart and/or block diagram block or blocks.

These computer program instructions may also be stored in a computerreadable medium that can direct a computer, other programmable dataprocessing apparatus, or other devices to function in a particularmanner, such that the instructions stored in the computer readablemedium produce an article of manufacture including instructions whichimplement the function/act specified in the flowchart and/or blockdiagram block or blocks.

The computer program instructions may also be loaded onto a computer,other programmable data processing apparatus, or other devices to causea series of operational steps to be performed on the computer, otherprogrammable apparatus or other devices to produce a computerimplemented process such that the instructions which execute on thecomputer or other programmable apparatus provide processes forimplementing the functions/acts specified in the flowchart and/or blockdiagram block or blocks.

FIGS. 7 and 8 show an exemplary process flow 700 for performing aspectsof the present invention. The steps of FIGS. 7 and 8 can be implementedin the system shown in FIG. 5.

The flowchart in FIGS. 7 and 8 illustrate the architecture,functionality, and operation of possible implementations of systems,methods, and computer program products according to various embodimentsof the present invention. In this regard, each block in the flowchart orblock diagrams may represent a module, segment, or portion of code,which includes one or more executable instructions for implementing thespecified logical function(s). It should also be noted that, in somealternative implementations, the functions noted in the block may occurout of the order noted in the figures. For example, two blocks shown insuccession may, in fact, be executed substantially concurrently, or theblocks may sometimes be executed in the reverse order, depending uponthe functionality involved. It will also be noted that each block of theblock diagrams and/or flowchart illustration, and combinations of blocksin the block diagrams and/or flowchart illustration, can be implementedby special purpose hardware-based systems that perform the specifiedfunctions or acts, or combinations of special purpose hardware andcomputer instructions.

FIGS. 7 and 8 show an exemplary process flow 700 for storing an inputvalue in a storage cell (e.g., storage cell 105) in accordance withaspects of the invention. The process is performed in a sequence ofiterative write and read operations that each progressively adjust aninitial value stored in the storage cell through a number ofintermediate values to arrive at a target value selected for the cell.In the example shown in FIGS. 7 and 8, the process includes seveniterations, which include seven write operations and six readoperations, wherein the last iteration is solely a write operation.

In accordance with aspects of the invention, the first iteration writesa predetermined write value (e.g., 32 in decimal notation) to themidpoint of the resistance range of the storage cell. Doing so places afirst value in the center of the entire resistance range of the PCM cellstorage cell (e.g., storage cell 105) to precondition the storage cell.The subsequent write iterations iteratively adjust the initial writevalue though a number of intermediate values to the target value. Thatis, the subsequent, read operations determine if the resulting valueread from the storage cell is lower than a target value (i.e., the 6-bitvalue determined from the 2-bit input value). In accordance with aspectsof the invention, no comparison is made to see if the stored value is“close enough” to the target value. In other words, there is nodetermination of whether the stored value is within a predeterminedrange of the target value. Thus, in embodiments of the presentinvention, no verification step is necessary. However, in someembodiments, such a verification step can be included in the process.

Additionally, in accordance with aspects of the invention, after thefirst write iteration, each of the subsequent write iterationscorrespond to a respective one of the bits in the write value. Inembodiments, each write iteration after the first determines therespective one of the plurality of bits. For example, for a 6-bit targetvalue, a write process can include seven iterations, wherein the firstiteration writes an initial, predetermined value and the six subsequentiterations set a respective bit of the target value by iterativelymodifying the initial value. The modification can be performed in abit-wise order from the most significant bit to the least significantbit. That is, the second pass determines the most significant bit (i.e.,bit 5); the third pass determines the next most significant bit (i.e.,bit 4); the fourth pass determines the next most significant bit (i.e.,bit 3); the fifth pass determines the next most significant bit (i.e.,bit 2); the sixth pass determines the next most significant bit (i.e.,bit 1); and the seventh pass determines the least significant bit (i.e.,bit 0).

Referring back to FIG. 7, at step 701 a multi-bit PCM circuit (e.g.,system 500) receives input value (e.g., 10 in binary notation) viainput/output pins (e.g., input/output pins 503). At step 702, thecircuit translates the input value to a target value to be stored in astorage cell based on the input value (e.g., in the manner describedwith respect to FIGS. 5 and 6). In embodiments, the input value is a2-bit number and the multi-bit PCM circuit (e.g., using write controller505) pads the input value with predefined bits to form a 6-bit value.For example, the pad value may be 1000 in binary, such that the inputvalues 00, 01, 10, and 11 are translated to write values of 001000,011000, 101000, and 111000, respectively.

At step 703, the circuit begins a first write iteration 703, which maybe triggered by a write enable signal (e.g., write enable signal 339)from a memory controller. At step 705, the circuit sets the write valuecorresponding to the middle of the resistance range of the storage cell.In embodiments, the resistance range is 000000 to 111111 (i.e. 0 to 63in decimal notation) and, as such, the middle of storage range is 100000(i.e., 32 in decimal notation).

At step 707, the circuit (e.g., using write head 300) writes the targetvalue of 100000 to the storage cell (e.g., in the manner described withrespect to FIGS. 3 and 4). At step 708, the circuit (e.g., using readhead 100) reads the stored value in the storage cell that resulted fromthe write operation of the first write iteration at step 707 (e.g., inthe manner described with respect to FIGS. 1 and 2). Due to physical andoperational variations among storage cells, the read value may not beequal to the write value. For example, the read value in this examplemay be 100101 compared to the write value of 100000.

At step 709, the circuit begins a second write iteration, which may betriggered by an end of the first write operation. The second writeiteration modifies the write value from step 705 based on the read valuefrom step 707 and the target value from step 702. More specifically, atstep 711, the circuit (e.g., using the write head controller 505)determines whether the read value from step 708 (e.g., 100101) isgreater than or equal to the target value from step 702 (i.e., 101000).In embodiments, the comparison uses only the three most significant bitsof the read value and pads them with zeros to provide a 6-bit value forcomparison. That is, the circuit can determine whether the three mostsignificant bits of read value (e.g., 100101) is greater than or equalto the target value (i.e., 101000).

At step 713, if the circuit determines in step 711 that the read valueis not greater than or equal to the target value, then the circuitincreases the write value from step 707. In embodiments, the two mostsignificant bits of the value from step 707 are incremented. Forexample, the write word from step 707 can be set to 110000. Otherwise atstep 715, if the circuit determines in step 711 that the read value isgreater than or equal to the target value, then the circuit decreasesthe write value from step 707. In embodiments, the two most significantbits of the value from step 707 are decremented. For example, the writeword from step 707 can be set to 010000.

At step 717, the circuit writes the write value determined at step 713or step 715 to the storage cell. At step 719, the circuit reads thevalue written in the storage cell at step 717. At step 721, the circuitbegins a third write iteration. The third write iteration modifies thewrite value from step 717 based on the read value from step 719 and thetarget value from step 702. More specifically, at step 723, the circuitdetermines whether the read value is greater than or equal to the targetvalue. In embodiments, the comparison uses only the three mostsignificant bits of the read value and pads them with zeros to provide a6-bit value for comparison. For example, assuming the process followedthe path including step 713, the read value can be 110101. Accordingly,at step 723, it can be determined whether the three most significantbits of the read value (e.g., 110101) are greater than the three mostsignificant bits of the target value (e.g., 101000).

At step 725, if the circuit determines in step 723 that the read valuefrom step 719 is not greater than or equal to the target value, then thecircuit increases the write value from step 717. In embodiments, thenext two most significant bits of the value from step 717 areincremented. For example, the write value from step 717 can be set toX11000 (wherein X corresponds to the respective bit of the write valuefrom step 717). Otherwise at step 727, if the circuit determines in step723 that the read value from step 719 is greater than or equal to thetarget value, then the circuit decreases the write value from step 717.In embodiments, the next two most significant bits of the value fromstep 717 are decremented. For example, the write value from step 717 canbe set to X01000 (wherein X corresponds to the respective bit of thewrite value from step 717).

At step 729, the circuit writes the write value determined at step 725or step 727 to the storage cell. At step 731, the circuit reads thevalue written in the storage cell at step 729. At step 735, the circuitbegins a fourth write iteration. The fourth write iteration modifies thewrite value at step 729 based on the read value from step 731 and thetarget value from step 702. More specifically, at step 735, the circuitdetermines whether the read value is greater than or equal to the targetvalue. In embodiments, the comparison uses only the three mostsignificant bits of the read value and pads them with zeros to provide a6-bit value for comparison. For example, assuming the process followedthe path including step 727, the read value can be 101101. Accordingly,at step 735, it can be determined whether the three most significantbits of the read value (e.g. 101101) are greater than the three mostsignificant bits of the target value (e.g., 101000).

At step 737, if the circuit determines that the read value from step 731is not greater than or equal to the target value, then the circuitincreases the write value from step 729. In embodiments, the next twomost significant bits of the value from step 729 are incremented. Forexample, the write value from step 729 can be set to XX1000 (wherein XXcorrespond to the respective bits of the write value from step 729).Otherwise at step 739, if the circuit determines in step 735 that theread value from step 731 is greater than or equal to the target value,then the circuit decreases the write value from step 729. Inembodiments, the next two most significant bits of the value from step729 are decremented. For example, the write value from step 729 can beset to XX0100 (wherein XX correspond to the respective bits of the writevalue from step 729).

At step 741, the circuit writes the write value determined at step 737or step 739 to the storage cell. FIG. 8 continues the process 700 shownin from point A. At step 743, the circuit reads the value written in thestorage cell at step 741. At step 745, the circuit begins a fifth writeiteration. The fifth write iteration modifies the write value from step741 based on the read value from step 743 and the target value from step702. More specifically, at step 747, the circuit determines whether theread value from step 743 is greater than or equal to the target value(e.g., 101000). In embodiments, the comparison uses only the three mostsignificant bits of the read value and pads them with zeros to provide a6-bit value for comparison. For example, assuming the process followedthe path including step 739, the read value can be 100111. Accordingly,at step 747, it can be determined whether the three most significantbits of the read value (e.g. 101000) are greater than the three mostsignificant bits of the target value (e.g., 101000).

At step 749, if the circuit determines that the read value from step 743is not greater than or equal to the target value from step 702, then thecircuit increases the write value from step 741. In embodiments, thenext two most significant bits of the value from step 741 areincremented. For example, the write value from step 741 can be set toXXX110 (wherein XXX correspond to the respective bits of the write valuefrom step 741). Otherwise at step 751, if the circuit determines in step747 that the read value from step 743 is greater than or equal to thetarget value from step 702, then the circuit decreases the write valuefrom step 741. In embodiments, the next two most significant bits of thevalue from step 741 are decremented. For example, the write value fromstep 741 can be set to XXX010 (wherein XXX correspond to the respectivebits of the write value from step 741).

At step 753, the circuit writes the write value determined at step 749or step 751 to the storage cell. At step 755, the circuit reads thevalue written in the storage cell at step 753. At step 757, the circuitbegins a sixth write iteration. The sixth write iteration modifies thewrite value from step 753 based on the read value from step 755 and thetarget value from step 702. More specifically, at step 759, the circuitdetermines whether the read value is greater than or equal to the targetvalue (i.e., 101000). In embodiments, the comparison uses only the threemost significant bits of the read value and pads them with zeros forcomparison. For example, assuming the process followed the pathincluding step 749, the read value can be 100111. Accordingly, at step759, it can be determined whether the three most significant bits of theread value (e.g. 100111) are greater than the three most significantbits of the target value (e.g., 101000).

At step 761, if the circuit determines that the read value from step 755is not greater than or equal to the target value from step 702, then thecircuit increases the write value from step 753. In embodiments, thenext two most significant bits of the value from step 753 areincremented. For example, the write value from step 753 can be set toXXXX11 (wherein XXXX correspond to the respective bits of the writevalue from step 753). Otherwise at step 763, if the circuit determinesthat the read value from step 755 is greater than or equal to the targetvalue from step 702, then the circuit decreases the write value fromstep 753. In embodiments, the next two most significant bits of thevalue from step 753 are decremented. For example, the write value fromstep 753 can be set to XXXX01 (wherein XXXX correspond to the respectivebits of the write value from step 753).

At step 765, the circuit writes the write value determined at step 761or step 763 to the storage cell. At step 767, the circuit reads thevalue written in the storage cell at step 765. At step 769, the circuitbegins a seventh write iteration. Notably, in embodiments of theinvention, the final write iteration does not include a correspondingread iteration. The seventh write iteration modifies the write value atstep 765 based on the read value from step 767 and the target value fromstep 702. More specifically, at step 771, the circuit determines whetherthe read value is greater than or equal to the target value (i.e.,101000). In embodiments, the comparison uses only the three mostsignificant bits of the read value and pads them with zeros forcomparison. For example, assuming the process followed the pathincluding step 761, the read value can be 101000. Accordingly, at step771, it can be determined whether the three most significant bits of theread value (e.g. 101000) are equal to the three most significant bits ofthe target value (e.g., 101000).

At step 773, if the circuit determines that the read value from step 767is not greater than or equal to the target value from step 702, then thecircuit increases the write value from step 765. In embodiments, theleast significant bit of the value from step 765 is incremented. Forexample, the write value from step 765 can be set to XXXXX1 (whereinXXXXX correspond to the respective bits of the write value from step765). Otherwise at step 775, if the circuit determines that the readvalue from step 767 is greater than or equal to the target value fromstep 702, then the circuit decreases the write value from step 765. Inembodiments, the least significant bit of the value from step 765 isdecremented. For example, the write value from step 765 can be set toXXXXX0 (wherein XXXXX correspond to the respective bits of the writevalue from step 765). At step 777, the circuit writes the write valuedetermined at step 773 or step 775 to the storage cell. After step 777,the process 700 ends.

In the process 700, the input value of 10 (i.e., 2 in decimal notation)is used to generate the target value 101000 (i.e., 40 in decimalnotation). In accordance with aspects of the invention, a number ofiterations are used to write the target value to accommodate variationsin different storage cells. That is, in the initial iteration (i.e., thefirst write iteration 703) the write value is always the same value thatis selected to be the middle of the resistance range of the storage cell(e.g., 32 of 64 resistance states). Subsequent iterations tune theinitial write value such that the value written in the final iteration(i.e., the seventh iteration 769) will result in a value stored in thecell that, when subsequently read, will result in a read value that isthe same or substantially the same as the target value. Accordingly, thecircuit reliably outputs the input value.

In the example shown in FIGS. 7 and 8, process 700 includes seveniterations, which include seven write operations and six read operationsto store a two-bit input value. Embodiments of the invention may useother numbers of iterations to store different numbers of bits. Further,in embodiments, “direct writing” of values can be used in combinationwith process 700. For example, embodiments can use a single write step(e.g. step 707) to write the lowest input value (i.e., 00) and thehighest input value (i.e., 11) without multiple iterations. In otherwords, the input value 00 can be written in a single write step (e.g.,step 707) to the lowest possible target value of 000000. Similarly, theinput value 11 bit can be written in a single write step to the highestpossible target value of 1111111. Input values other than 00 or 11 wouldstill be written using process 700. By performing such a “direct write”of the 00 and 11 inputs, embodiments of the invention would decrease thetotal write time.

FIG. 9 illustrates a write process (e.g., process 700) by an exemplarysystem (e.g., system 500) in accordance with aspects of the invention.FIG. 9 shows a relationship between timing of signals in an exemplarywrite process (e.g., process 700). The signals includes the write enablesignal 339, the read enable signal 550, write-reference voltage 318,write signal 345, read reference signal 118, and output signal 150. Thewrite enable signal 339 is a control logic signal received from, forexample, a memory controller that activates the write process. Inembodiments, the write process includes seven write iterations (e.g.,steps 703, 709, 721, 733, 745, 757, and 769). The write enable signal339 in a high logic state throughout the write process, including eachwrite iteration.

The read enable signal 550 is a control logic signal generated by thesystem (e.g., read head 100) that activates read operations. Forexample, where the write process includes seven write iterations,including six read operations (e.g., steps 708, 719, 731, 743, 755, and767), the system can set the read signal 550 to a logic high state totrigger the read operations between each of the write operations (e.g.,steps 707, 717, 729, 741, 753, 765, and 777). As shown in FIG. 9, theread signal 550 is placed in high logic state to start the readoperations after completion of the write operations.

The write-reference voltage 318 is a time-varying signal that ramps(e.g. increases) during the period of each write iteration to providecurrent that is used to write values to a storage cell (e.g., storagecell 105). For example, in an exemplary write operation (e.g., processflow 700) including seven write iterations, there are seven write ramps931 . . . 937 of the write signal corresponding to each write operation(e.g., steps 707, 717, 729, 741, 753, 765, and 777). Each of the writeramps 931 . . . 937 increase a current that can be driven into thestorage cell.

The write signal 345 is a control logic signal determined by the systemthat triggers the writing of a value to the storage cell at a particulartime corresponding to a value of the write-reference voltage 318. Forexample, when the write-reference voltage 318 ramps up (e.g., from 0 to63) over time (e.g., time increments 0-63), the write signal pulses(e.g., from a low logic state to a high logic state) at a time (e.g.,time increment 32) to program a corresponding resistance state of thestorage cell, which represents information (e.g., 32). In accordancewith aspects of the invention, each of the write operations (e.g., steps707, 717, 729, 741, 753, 765, and 777) generates a pulse 941 . . . 947at a particular time during a corresponding write ramp 931 . . . 937such that the current voltage of the write ramps 931 . . . 937 drivescurrent into the storage cell to change the resistive state of thestorage cell.

The read-reference voltage 118 is a time-varying signal that increases(i.e., ramps) over time to provide current used to read values from thestorage cell. In embodiments, there are six read ramps 951 . . . 956 ofthe read control signal corresponding to each read operations (e.g.,steps 708, 719, 731, 743. 755, and 767). Each increment of time andvoltage (e.g., steps) of the read-reference voltage 118 corresponds to adiscrete resistance state of the storage cell. For example, theread-reference voltage 118 can increase in 64 steps that correspond to64 discrete resistance states of the storage cell.

The read output signal 150 is a logic signal that indicates the value ofthe storage cell during a read operation (e.g., steps 708, 719, 731,743. 755, and 767). The read output signal 150 change logic states(e.g., from a high logic state to a low logic state) at the time whenthe ramped voltage of the read reference signal 118 is the same as thevoltage across the storage cell. As such, the time the read outputsignal 150 changes state indicates the value stored in the storage cell.For example, in FIG. 9, the read output signal changes states at 961 . .. 966.

FIGS. 10-22 illustrate an exemplary write process (e.g., process 700)for a multi-bit PCM system (e.g., system 500) in accordance with aspectsof the invention. In the present example, the system stores 2-bits ofinformation representing the binary value 10, which is stored in asingle storage cell of the multi-bit PCM. These 2-bits may be part of alarger binary value that is stored in using several storage cells of themulti-bit PCM. For example, an 8-bit word can be stored in four storagecells.

In accordance with aspects of the invention, the received input value istranslated to a value that corresponds to a resistance state of thestorage cell (e.g., using the process of FIG. 6). In the preset example,the storage cell can assume any of 64 resistance states (e.g. 0-63 indecimal, or 000000-111111 in binary). Accordingly, the received 2-bitvalue is translated to a G-bit value that corresponds to one of the 64resistance state. As the present example stores input in four portionsof the 64 resistance states to leave margin for error, the circuit canpad the 2-bit input value with predefined bits to form a 6-bit value.That is, the pad value may be 1000 in binary, such that the input values10 is translated to a target values of 101000 (i.e., 40), whichcorrespond to one of the resistive states of the storage cell.

In accordance with aspects of the invention, the system sets the targetvalue of 101000 using seven iterations, wherein a first operation storesan initial write value at the center of resistance range of the storagecell. Since there are 64 resistance states having values ranging from0-63 (i.e., 000000 to 111111 in binary notation) in this example, thecenter of the resistance range is 32 (i.e., 100000 in binary notation).Subsequent write operations progressively modify (e.g., tune) thisinitial write value 100000 such that it is the same or substantially thesame as the target value of 101000.

FIGS. 10 and 11 illustrate a first write iteration (e.g., step 703).Referring to FIG. 10, the system stores the initial write value to100000 (e.g., step 705), which is at the center of the resistance rangeof the storage cell. Accordingly, in a first write operation (e.g., step707), the system triggers the write signal 345 such that it pulses at32nd time increment when a ramped write-reference voltage 318 is at a32nd step (i.e. step 100000) that corresponds to the a 32nd resistancestate of the storage cell.

FIG. 11 shows a first read operation (e.g., step 708) of the exemplarywrite process. As shown the read-reference signal 118 is ramped from aninitial voltage step 0 to a final voltage step 63. The ramping of theread reference signal 118 can be triggered by a read enable signal(e.g., read control signal 550) at the end of the first write operation.When read-reference voltage 118 is the same as the voltage across thestorage cell due to the stored value, the read output signal 150switches logic states. In this example, the read output signal 150switches states at step 37 of the read reference signal. Accordingly,the multi-bit PCM determines that the value of storage cell is 37 (i.e.,100101 in binary). This value may be obtained from a counter synced withthe step of the read reference signal 118. The difference between thewrite value of 32 and the read value of 37 may be due physical andoperational variations among storage cells such that the voltage of thewrite reference signal 318 at step 32 does not align with thecorresponding resistance of the storage cell.

FIGS. 12 and 13 illustrate a second write iteration (e.g., step 709).Here, the system stores the write value to 48 (i.e., 110000). This writevalue is determined based on the difference between the read value inFIG. 11 (i.e. 100101) and the target value (i.e., 101000). That is, inaccordance with aspects of the invention, the system compares the threemost significant bits of the read value (i.e., 100101) with those of thetarget value (i.e., 101000). Since the read value is less than thetarget value, the system increases the write value from 100000 (i.e.,32) to 1100000 (i.e., 48) by incrementing the two-most significant bits,which increase the value by 16 (i.e., from 32 to 48).

Referring to FIG. 12, the system stores the determined write value of 48(i.e., 110000) in the storage cell. Accordingly, in a second writeoperation (e.g., step 717), the system triggers the write signal 345such that it pulses at 48th time increment (e.g., 110000) when a rampedwrite-reference voltage 318 is at a 48th step (i.e. step 110000) thatcorresponds to the a 48th resistance state of the storage cell. FIG. 13shows a second read operation (e.g., step 719) of the exemplary writeprocess. In this read operation, the read output signal 150 (e.g.,output signal 150) switches states at step 53 of the read referencesignal 118. Accordingly, the system determines that the value of storagecell is 53 (i.e., 110101 in binary).

FIGS. 14 and 15 illustrate a third write iteration (e.g., step 721). Inthis iteration, the system sets the write value to 40 (i.e., 101000).The write value is determined based on the difference between the value110101 read in FIG. 13 (i.e. 53) and the target value 101000 (i.e., 40).That is, since the three most significant bits of the read value (i.e.,110000) is not less than those of the target value (i.e., 101000), thesystem decreases the write value from 110000 (i.e., 48) to 101000 (i.e.,40) by decrementing the second and third-most significant bits, whichdecreased the write value by 8.

As shown in FIG. 14, the third write iteration stores the determinedwrite value of 40 (i.e., 101000) in the storage cell. That is, thesystem triggers the write signal 345 such that it pulses at 40th timeincrement when a ramped write-reference voltage 318 is at a 40th step(i.e. step 101000) that corresponds to the a 40th resistance state ofthe storage cell. FIG. 15 shows a third read operation (e.g., step 731)of the exemplary write process. In this read operation, the read outputsignal 150 switches states at step 45 of the read reference signal 118.Accordingly, the system determines that the value of storage cell is 45(i.e., 101101 in binary).

FIGS. 16 and 17 illustrate a fourth write iteration (e.g., step 733).Here, the system sets the write value to 36 (i.e., 100100), which isdetermined based on the difference between the value 101101 read in FIG.15 (i.e. 45) and the target value 101000 (i.e., 40). That is, since thethree most significant bits of the read value (i.e., 101101) are greaterthan or equal to three most significant bits of the target value (i.e.,101000), the system decreases the previous write value from 101000(i.e., 40) to 100100 (i.e., 36) by holding the first and second mostsignificant bits, decreasing the third most significant bit, andincreasing the fourth most significant bit, which decreases the targetvalue by 4.

As shown in FIG. 16, the fourth write iteration (e.g., step 741) storesthe determined write value of 36 (i.e., 100100) in the storage cell.That is, the system triggers the write signal 345 such that it pulses at36th time increment when a ramped write-reference voltage 318 is at a36th step (i.e. step 100100) that corresponds to the a 36th resistancestate of the storage cell. FIG. 17 shows a fourth read operation (e.g.,step 743) of the exemplary write process. Here, the read output signal150 switches states at step 41 of the read reference signal 118.Accordingly, the system determines that the value of storage cell is 41(i.e., 101001 in binary).

FIGS. 18 and 19 illustrate a fifth write iteration (e.g., step 745). Inthis case, the system sets the write value to 34 (i.e., 100010), whichis determined based on the difference between the value read 101001 inFIG. 17 (i.e. 41) and the target value 101000 (i.e., 40). That is, thethree most significant bits of the read value (i.e., 101001) are greaterthan or equal to those of the target value (i.e., 101000). Thus, thesystem decreases the previous write value from 100100 (i.e., 36) to100010 (i.e., 34), which decreases the target value by 2.

FIG. 18 shows the fifth write operation (e.g., step 753), which storesthe determined write value of 34 (i.e., 100100) in the storage cell.That is, the system triggers the write signal 345 such that it pulses at34th time increment when a ramped write-reference voltage 318 is at a34th step (i.e. step 100010) that corresponds to the a 34th resistancestate of the storage cell. FIG. 19 shows a fifth read operation (e.g.,step 755) of the exemplary write process. In this read operation, theread output signal 150 switches states at step 39 of the read referencesignal 118. Accordingly, the system determines that the value of storagecell is 39 (i.e., 100111 in binary).

FIGS. 20 and 21 illustrate a sixth write iteration (e.g., step 757).Here, the system sets the write value to 35 (i.e., 100011), which isdetermined based on the difference between the value read 100111 in FIG.19 (i.e. 39) and the target value 101000 (i.e., 40). That is, the threemost significant bits of the read value (i.e., 100111) are not greaterthan or equal to target value (i.e., 101000). Thus, the system increasesthe previous write value from 100010 (i.e., 34) to 100011 (i.e., 35), bykeeping the first, second, third, and fourth most significant bits andincreasing the fifth-most significant bit of the previous write word,which increases the target value by 1.

FIG. 20 shows the sixth write operation (e.g., step 765), which storesthe determined write value of 35 (i.e., 100011) in the storage cell.That is, the system triggers the write signal 345 such that it pulses at35th time increment when a ramped write-reference voltage 318 is at a 35step (i.e. step 100011) that corresponds to the 35 resistance state ofthe storage cell. FIG. 21 shows a sixth read operation (e.g., step 767)of the exemplary write process. The read output signal 150 switchesstates at step 40 of the read reference signal 118. Accordingly, thesystem determines that the value of storage cell is 40 (i.e., 101000 inbinary). Notably, in this iteration, the read value equals the targetvalue. Nevertheless, in accordance with aspects of the invention, thewrite operation continues since it does not rely on a difference betweenthe target value and the read value to stop the process. Instead, itrelies on the fact that the number of write iterations adjusts thestored value to a correct result.

FIG. 22 shows a seventh, and final, write iteration (e.g., step 769) ofthe exemplary process. In this iteration, the system sets the writevalue to 34 (i.e., 100010), which is determined based on the differencebetween the value read 101000 in FIG. 21 (i.e. 40) and the target value101000 (i.e., 40). That is, the three most significant bits of the readvalue (i.e., 101000) are greater than or equal to target value (i.e.,101000). Thus, the system decreases the previous write value from 100011(i.e., 35) to 100010 (i.e., 34), by keeping the first, second, third,and fourth most significant bits and decreasing the fifth-mostsignificant bit of the previous write word, which decreases the targetvalue by 1. Accordingly, in the seventh write iteration (e.g., step 777)shown in FIG. 22, the system triggers the write signal 345 such that itpulses at 34th time increment when a ramped write-reference voltage 318is at a 34th step (i.e. step 100010) that corresponds to the a 34thresistance state of the storage cell.

FIG. 23 is a flow diagram of a design process used in semiconductordesign, manufacture, and/or test. FIG. 23 shows a block diagram of anexemplary design flow 2300 used for example, in semiconductor IC logicdesign, simulation, test, layout, and manufacture. Design flow 2300includes processes, machines and/or mechanisms for processing designstructures or devices to generate logically or otherwise functionallyequivalent representations of the design structures and/or devicesdescribed above and shown in FIGS. 1, 3, and 5. The design structuresprocessed and/or generated by design flow 2300 may be encoded onmachine-readable transmission or storage media to include data and/orinstructions that when executed or otherwise processed on a dataprocessing system generate a logically, structurally, mechanically, orotherwise functionally equivalent representation of hardware components,circuits, devices, or systems. Machines include, but are not limited to,any machine used in an IC design process, such as designing,manufacturing, or simulating a circuit, component, device, or system.For example, machines may include: lithography machines, machines and/orequipment for generating masks (e.g., e-beam writers), computers orequipment for simulating design structures, any apparatus used in themanufacturing or test process, or any machines for programmingfunctionally equivalent representations of the design structures intoany medium (e.g. a machine for programming a programmable gate array).

Design flow 2300 may vary depending on the type of representation beingdesigned. For example, a design flow 2300 for building an applicationspecific IC (ASIC) may differ from a design flow 2300 for designing astandard component or from a design flow 2300 for instantiating thedesign into a programmable array, for example a programmable gate array(PGA) or a field programmable gate array (FPGA) offered by Altera® Inc.or Xilinx® Inc.

FIG. 23 illustrates multiple such design structures including an inputdesign structure 2320 that is preferably processed by a design process2310. Design structure 2320 may be a logical simulation design structuregenerated and processed by design process 2310 to produce a logicallyequivalent functional representation of a hardware device. Designstructure 2320 may also or alternatively comprise data and/or programinstructions that when processed by design process 2310, generate afunctional representation of the physical structure of a hardwaredevice. Whether representing functional and/or structural designfeatures, design structure 2320 may be generated using electroniccomputer-aided design (ECAD) such as implemented by a coredeveloper/designer. When encoded on a machine-readable datatransmission, gate array, or storage medium, design structure 2320 maybe accessed and processed by one or more hardware and/or softwaremodules within design process 2310 to simulate or otherwise functionallyrepresent an electronic component, circuit, electronic or logic module,apparatus, device, or system such as those shown in FIGS. 1, 3, and 5.As such, design structure 2320 may comprise files or other datastructures including human and/or machine-readable source code, compiledstructures, and computer-executable code structures that when processedby a design or simulation data processing system, functionally simulateor otherwise represent circuits or other levels of hardware logicdesign. Such data structures may include hardware-description language(HDL) design entities or other data structures conforming to and/orcompatible with lower-level HDL design languages such as Verilog andVHDL, and/or higher level design languages such as C or C++.

Design process 2310 preferably employs and incorporates hardware and/orsoftware modules for synthesizing, translating, or otherwise processinga design/simulation functional equivalent of the components, circuits,devices, or logic structures shown in FIGS. 1, 3, and 5 to generate anetlist 2380 which may contain design structures such as designstructure 2320. Netlist 2380 may comprise, for example, compiled orotherwise processed data structures representing a list of wires,discrete components, logic gates, control circuits, I/O devices, models,etc. that describes the connections to other elements and circuits in anintegrated circuit design. Netlist 2380 may be synthesized using aniterative process in which netlist 2380 is resynthesized one or moretimes depending on design specifications and parameters for the device.As with other design structure types described herein, netlist 2380 maybe recorded on a machine-readable data storage medium or programmed intoa programmable gate array. The medium may be a non-volatile storagemedium such as a magnetic or optical disk drive, a programmable gatearray, a compact flash, or other flash memory. Additionally, or in thealternative, the medium may be a system or cache memory, buffer space,or electrically or optically conductive devices and materials on whichdata packets may be transmitted and intermediately stored via theInternet, or other networking suitable means.

Design process 2310 may include hardware and software modules forprocessing a variety of input data structure types including netlist2380. Such data structure types may reside, for example, within libraryelements 2330 and include a set of commonly used elements, circuits, anddevices, including models, layouts, and symbolic representations, for agiven manufacturing technology (e.g., different technology nodes, 32 nm,45 nm, 90 nm, etc.). The data structure types may further include designspecifications 2340, characterization data 2350, verification data 2360,design rules 2370, and test data files 2385 which may include input testpatterns, output test results, and other testing information. Designprocess 2310 may further include, for example, standard mechanicaldesign processes such as stress analysis, thermal analysis, mechanicalevent simulation, process simulation for operations such as casting,molding, and die press forming, etc. One of ordinary skill in the art ofmechanical design can appreciate the extent of possible mechanicaldesign tools and applications used in design process 2310 withoutdeviating from the scope and spirit of the invention. Design process2310 may also include modules for performing standard circuit designprocesses such as timing analysis, verification, design rule checking,place and route operations, etc.

Design process 2310 employs and incorporates logic and physical designtools such as HDL compilers and simulation model build tools to processdesign structure 2320 together with some or all of the depictedsupporting data structures along with any additional mechanical designor data (if applicable), to generate a second design structure 2390.

Design structure 2390 resides on a storage medium or programmable gatearray in a data format used for the exchange of data of mechanicaldevices and structures (e.g. information stored in a IGES, DXF,Parasolid XT, JT, DRG, or any other suitable format for storing orrendering such mechanical design structures). Similar to designstructure 2320, design structure 2390 preferably comprises one or morefiles, data structures, or other computer-encoded data or instructionsthat reside on transmission or data storage media and that whenprocessed by an ECAD system generate a logically or otherwisefunctionally equivalent form of one or more of the embodiments of theinvention shown in FIGS. 1, 3, and 5. In one embodiment, designstructure 2390 may comprise a compiled, executable HDL simulation modelthat functionally simulates the devices shown in FIGS. 1, 3, and 5.

Design structure 2390 may also employ a data format used for theexchange of layout data of integrated circuits and/or symbolic dataformat (e.g. information stored in a GDSII (GDS2), GL1, OASIS, mapfiles, or any other suitable format for storing such design datastructures). Design structure 2390 may comprise information such as, forexample, symbolic data, map files, test data files, design contentfiles, manufacturing data, layout parameters, wires, levels of metal,vias, shapes, data for routing through the manufacturing line, and anyother data required by a manufacturer or other designer/developer toproduce a device or structure as described above and shown in FIGS. 1,3, and 5. Design structure 2390 may then proceed to a stage 2395 where,for example, design structure 2390: proceeds to tape-out, is released tomanufacturing, is released to a mask house, is sent to another designhouse, is sent back to the customer, etc.

The method as described above is used in the fabrication of integratedcircuit chips. The resulting integrated circuit chips can be distributedby the fabricator in raw wafer form (that is, as a single wafer that hasmultiple unpackaged chips), as a bare die, or in a packaged form. In thelatter case the chip is mounted in a single chip package (such as aplastic carrier, with leads that are affixed to a motherboard or otherhigher level carrier) or in a multichip package (such as a ceramiccarrier that has either or both surface interconnections or buriedinterconnections). In any case the chip is then integrated with otherchips, discrete circuit elements, and/or other signal processing devicesas part of either (a) an intermediate product, such as a motherboard, or(b) an end product. The end product can be any product that includesintegrated circuit chips, ranging from toys and other low-endapplications to advanced computer products having a display, a keyboardor other input device, and a central processor.

The descriptions of the various embodiments of the present inventionhave been presented for purposes of illustration, but are not intendedto be exhaustive or limited to the embodiments disclosed. Manymodifications and variations will be apparent to those of ordinary skillin the art without departing from the scope and spirit of the describedembodiments. The terminology used herein was chosen to best explain theprinciples of the embodiments, the practical application or technicalimprovement over technologies found in the marketplace, or to enableothers of ordinary skill in the art to understand the embodimentsdisclosed herein.

What is claimed:
 1. A phase change memory system comprising: a storagecell, a read head, and a write head, wherein: the read head retrievesinformation stored in the storage cell using a read-reference voltagethat incrementally ramps over a read period; and the write head storesinformation in the storage cell using a write-reference voltage thatincrementally ramps over a write period.
 2. The system of claim 1,wherein the read head comprises: a digital-to-analog converter thatestablishes the read-reference voltage; and a comparator that comparesthe read-reference voltage to a voltage across the storage cell.
 3. Thesystem of claim 1, wherein the write head comprises: a digital-to-analogconverter that establishes the write-reference voltage; and a circuitthat generates a write current based on the write-reference voltage. 4.The system of claim 1, further comprising: a write head controllerconfigured to determine a time interval of the write-reference voltageto trigger the write head based on an input value; and a read headcontroller configured to determine a value stored at the storage cellbased upon a time interval of the read-reference voltage at which theread head triggers.
 5. The system of claim 1, wherein the read head andthe write head are devoid of an analog-to-digital converter.
 6. Thesystem of claim 1, wherein the read head includes a sense comparatorhaving a first input, a second input, and an output.
 7. The system ofclaim 6, wherein the sense comparator is configured to change logicstates from a high logic state to a low logic state when the first inputand the second input are a same or substantially the same.
 8. The systemof claim 7, wherein the first input of the sense comparator receives aread-reference voltage that is ramped over a predetermined read period,the read-reference voltage is generated by a controllable currentsource.
 9. The system of claim 8, wherein the read-reference voltage isramped in 64 increments having a respective voltage level thatcorresponds to 64 different resistance states of the storage cell. 10.The system of claim 9, wherein: the storage cell is associated with abit line (“BL”), a word line (“WL”), a master bit line (“MBL”), and acolumn decoder line (“CD”); the word line selectively connects thestorage cell to ground voltage via a first transistor; the columndecoder selectively connects the storage cell and the bit line to themaster bit line via a second transistor; the storage cell is selectedfor reading by the bit line, the word line, and the column decoder suchthat a voltage corresponding to the value of the storage cell isproduced on the master bit line due to the voltage across the storagecell based on its resistance state.